Kosowski became renowned in Poland after his performances for Wisła Kraków in the 2002–03 UEFA Cup matches.

He moved to 1. FC Kaiserslautern in Germany in 2003 and then on to Southampton FC where he played the 2005–06 season on a year-long loan and was recalled to the Polish national team. He scored once in the league for Southampton, in a 1–1 draw with Hull City.[1]

After being on loan at A.C. Chievo Verona, Kosowski returned to Wisła Kraków and played very well for the first half of the season as the player with most assists in the first half of the league campaign after setting up nine goals in 13 games. However on 14 January 2008, Wisła Kraków and Kosowski mutually voided the remainder of his contract as a result of not coming to a compromise on a new deal.

Kosowski signed a two and a half year contract with Spanish second division team Cádiz CF on 25 January 2008. Cadiz finished 20th in Segunda Division (2007–08) and were relegated to Segunda Division B.